Choosing a carburetor
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==Selecting carburetor size== | ==Selecting carburetor size== | ||
| − | The higher the engine can rev, the greater the required CFM. However, bottom end | + | The higher the engine can rev, the greater the required CFM. However, if the primaries are too large the drop in vacuum caused when the throttle is opened can hurt the bottom end and drivability. |
===Based on displacement and use=== | ===Based on displacement and use=== | ||
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==Selecting carburetor type== | ==Selecting carburetor type== | ||
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Opinions vary on the choice between vacuum and mechanical secondaries (aka "double pumpers"). However, generally speaking: | Opinions vary on the choice between vacuum and mechanical secondaries (aka "double pumpers"). However, generally speaking: | ||
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Modern day carburetors- regardless of the brand- are designed to accomplish the same basic things: Meter and atomize the fuel, mix it with air, and deliver this air/fuel mixture in the correct ratio to the engine to allow it to run efficiently over a broad range of load, conditions and engine speeds from idle to WOT. How these things are done by each particular brand of carb can differ somewhat. | Modern day carburetors- regardless of the brand- are designed to accomplish the same basic things: Meter and atomize the fuel, mix it with air, and deliver this air/fuel mixture in the correct ratio to the engine to allow it to run efficiently over a broad range of load, conditions and engine speeds from idle to WOT. How these things are done by each particular brand of carb can differ somewhat. | ||
| − | On the primary side the carb has the duty of supplying air/fuel to allow the engine to start up cold (choke), to idle, to accelerate the vehicle | + | On the primary side, the carb has the duty of supplying air/fuel to allow the engine to start up cold (choke), to idle (idle circuit), to accelerate the vehicle (accelerator pump circuit and the transition/main circuits) up to full throttle, where the secondary and/or power enrichment circuits are called on. Of the commonly used carbs on the market, the Q-jet probably has the most responsive primaries of them all. This is due to the relatively small size of the primaries along w/the triple boosters that enhance the vacuum signal to the circuits controlling the air/fuel mixture at the various loads/speeds. |
| − | The non mechanical secondary carbs | + | The non mechanical secondary carbs (Q-jet, Holley 4160'''*''', etc., Edelbrock/Carter to name a few) secondaries all work on a "as needed" basis. That's to say they won't (if properly adjusted) allow any more secondary opening than the engine actually needs. This is unlike a Holley double pumper mechanical secondary carb, which opens up the secondaries regardless of the engine demand, if you floor it. |
'''*'''The model 4160 is a vacuum secondary carb- w/few exceptions, one is the 0-4224 660 cfm "center squirter" carb, originally designed to fit on a dual four barrel intake end-to-end. Another is the 450 cfm 0-9776 Model 4160 mechanical secondary carb. It has no secondary accelerator pump. These carbs are designed to be used in a multiple carb application. They have no choke flap, they have side hung float bowls and single fuel inlet. The model 4150 can be either a vacuum secondary or a double pumper carb. | '''*'''The model 4160 is a vacuum secondary carb- w/few exceptions, one is the 0-4224 660 cfm "center squirter" carb, originally designed to fit on a dual four barrel intake end-to-end. Another is the 450 cfm 0-9776 Model 4160 mechanical secondary carb. It has no secondary accelerator pump. These carbs are designed to be used in a multiple carb application. They have no choke flap, they have side hung float bowls and single fuel inlet. The model 4150 can be either a vacuum secondary or a double pumper carb. | ||
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You may hear a Q-jet being called a 'mechanical secondary' carb. And to some extent this is true- the secondary throttle blades WILL open when you floor it if the choke is off. But- and this is an important 'but'- it's the air valve at the top of the carb that works on the 'as needed' basis. The Edelbrock/Carter is a similar design in that the secondary throttle blades will open if floored, but the Edelbrock/Carter uses a air door that works by the amount of air flowing into the secondary side and it too, won't give the engine any more airflow than it needs. | You may hear a Q-jet being called a 'mechanical secondary' carb. And to some extent this is true- the secondary throttle blades WILL open when you floor it if the choke is off. But- and this is an important 'but'- it's the air valve at the top of the carb that works on the 'as needed' basis. The Edelbrock/Carter is a similar design in that the secondary throttle blades will open if floored, but the Edelbrock/Carter uses a air door that works by the amount of air flowing into the secondary side and it too, won't give the engine any more airflow than it needs. | ||
| − | The vacuum secondary Holley uses a different approach to secondary actuation, where the secondary throttle blades are controlled by engine demand via the secondary vacuum diaphragm assembly. But like the Q-jet and Edelbrock/Carter carbs, the vacuum secondary Holley is also self-regulating. And because of that, the Edelbrock/Carter and Holley | + | The vacuum secondary Holley uses a different approach to secondary actuation, where the secondary throttle blades are controlled by engine demand via the secondary vacuum diaphragm assembly. But like the Q-jet and Edelbrock/Carter carbs, the vacuum secondary Holley is also self-regulating. And because of that, the Edelbrock/Carter and Holley vacuum secondary carbs are somewhat more forgiving if a too-large carb is used. |
I say ''somewhat'' more forgiving because even though a 750 Holley w/vacuum secondaries will self-regulate the secondaries, the primaries- what you drive on 99% of the time- are also larger on a 750 than a 600 Holley. And what that can mean is on a mild or smaller displacement engine the 750 cfm carb won't give as crisp of a throttle response and possibly less MPG and vacuum because of the bigger throttle bore and venturi. This is not an issue w/the Q-jet; its primaries are small and have primary booster rings to provide a strong vacuum signal. The 800 cfm Q-jet castings are used on 4.3L truck engines to give an idea of the flexibility of a Q-jet. | I say ''somewhat'' more forgiving because even though a 750 Holley w/vacuum secondaries will self-regulate the secondaries, the primaries- what you drive on 99% of the time- are also larger on a 750 than a 600 Holley. And what that can mean is on a mild or smaller displacement engine the 750 cfm carb won't give as crisp of a throttle response and possibly less MPG and vacuum because of the bigger throttle bore and venturi. This is not an issue w/the Q-jet; its primaries are small and have primary booster rings to provide a strong vacuum signal. The 800 cfm Q-jet castings are used on 4.3L truck engines to give an idea of the flexibility of a Q-jet. | ||
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